The invention relates generally to semiconductor device and integrated circuit fabrication and, in particular, to device structures and fabrication methods for heterojunction bipolar transistors.
A bipolar junction transistor is a three-terminal electronic device that includes an emitter, a collector, and an intrinsic base arranged between the emitter and collector. In an NPN bipolar junction transistor, the emitter and collector may be composed of n-type semiconductor material, and the intrinsic base may be composed of p-type semiconductor material. In a PNP bipolar junction transistor, the emitter and collector may be composed of p-type semiconductor material, and the intrinsic base may be composed of n-type semiconductor material. In operation, the base-emitter junction is forward biased, the base-collector junction is reverse biased, and the collector-emitter current may be controlled with the base-emitter voltage.
A heterojunction bipolar transistor is a variant of a bipolar junction transistor in which at least two of the collector, emitter, and intrinsic base are composed of semiconductor materials with different energy bandgaps, which creates heterojunctions. For example, the collector and/or emitter of a heterojunction bipolar transistor may be composed of silicon, and the base of a heterojunction bipolar transistor may be composed of silicon germanium (SiGe), which is characterized by a narrower band gap than silicon.
In some applications such as power amplifiers, the emitter of a heterojunction bipolar transistor may include multiple emitter fingers. An extrinsic base region is arranged between each pair of emitter fingers. These extrinsic base regions contribute to the parasitic capacitance of the device structure, which may adversely impact device performance.
Improved structures and fabrication methods for heterojunction bipolar transistors are needed.